With the increasing use and versatility of mobile devices, user authentication becomes increasingly important. Increasing amounts of personal information may be stored on or accessible by a mobile device. Moreover, mobile devices are increasingly being used to make purchases and perform other commercial transactions. Existing authentication methods typically involve the use of a password or passcode, which may be forgotten by a rightful user or used by an unauthorized person. Users of such devices prefer device interfaces that are fast, accurate, secure, easy to use, and consume minor portions of battery life.
In mobile devices, such as mobile phones, tablet computers and wearable devices, it is often desirable to protect the display and touch interfaces of the mobile devices with hard materials to improve the mechanical robustness and reliability. However, with ultrasonic sensor arrays for touch or fingerprint detection, a hard platen material can negatively impact the differential acoustic signal between a ridge and a valley of the finger, because the acoustic impedance of the hard platen material may be much higher compared to the ridges and valleys of the finger. With high acoustic impedance mismatches between the platen and a finger, both finger tissue and air reflect most of the acoustic energy back to the sensor array. In addition, there may be appreciable mismatches between the hard platen materials and the polymeric piezoelectric layers used in some ultrasonic sensor arrays.
Thus, it is desirable to improve signal levels of ultrasonic sensors to enable mobile devices to use hard cover layer materials and to provide a one-step user interface for a user to authenticate and activate a function of a mobile device.